Proton exchange membrane (PEM) fuel cells show great promise for future power generation applications due to their high efficiency and low emissions, but cost remains one of the chief obstacles to widespread commercialization. The platinum (Pt) catalyst, particularly at the cathode, constitutes a significant fraction of the overall cost of the fuel cell system, and much effort has therefore been directed toward increasing the Pt-specific power density, that is, the power output per gram of Pt. This goal can be accomplished either by replacing the precious metal with a lower-cost catalyst, or by optimizing the structure to utilize the Pt more effectively. Sputter deposition is a promising method for manufacturing catalyst layers with ultra-low Pt loadings, because the Pt loading, morphology and thickness can be precisely controlled, resulting in a uniform layer in a simple and scalable process.
In order to completely control the assembly of the three-phase boundary, where oxygen, electrons, and ions meet to react at a catalyst site, Pt sputtering needs to be combined with processing steps that control the overall electrode structure, and in particular the electrode porosity. Challenges are associated with the complexity of the involved processing steps and particularly with the bonding of fragile nanostructured electrodes to the membrane. For example, Hayase et al. have reported excellent results for a porous Si catalyst support formed by photolithographic patterning and subsequent wet etching, but the porous Si is brittle and therefore limits the clamping pressure that can be applied to the cell [M. Hayase, T. Kawase and T. Hatsuzawa, Electrochem. Solid-State Lett., 7, A231 (2004)].